A cellular communications system is based around cells or similar radio coverage areas. Examples of cellular telecommunications systems include standards such as, without limiting to these, GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), CDMA (Code Division Multiple Access) or the proposed WCDMA (Wideband CDMA), or UMTS (Universal Mobile Telecommunications System) or other third generation communication system and so on. In general, a cell coverage area of a telecommunication system can be defined as a certain geographically limited area covered by one or several base transceiver stations (BTS) serving user equipment (UE) or mobile stations (MS) via an air or radio interface and usually connected to a base station subsystem (BSS). Several cells connected to each other cover a larger geographical area, and thus several cells form together a larger geographical area, and cellular telecommunication network.
Each coverage area of the system can be controlled by an appropriate controller apparatus. The controller can be connected further to a gateway or linking apparatus, such as a gateway GPRS support node (GGSN) or gateway mobile switching center (GSMC), linking the cell to the other communication networks, such as to a PSTN (Public Switched telecommunications Network) or to a data network, such as to a X.25 based network or to a TCP/IP (Transmission Control Protocol/Internet Protocol) based network.
The user equipment (UE) (that will be referred to in the following as mobile station or MS) within one of the cells of the telecommunications systems is respectively controlled by the controller functionality of the cell. The MS may be controlled by only one controller at time. However, the MS may also be simultaneously controlled by several controllers. This may occur e.g. when the cells overlap or in so called soft handoff mode, where the MS may be in communication with two base stations and those base stations may be connected to different controllers, or when one controller is controlling another controller controlling the MS. One controller can be defined as the serving (main) controller whereas the others act as secondary controllers.
The mobile station communicates with an appropriate network controller and provides the controller with various types of information. For instance, when the mobile station changes from one cell to a new cell or when the mobile station is switched on in one of the cells after having been switched off or unreachable for a while, the mobile station will send a message containing a MS identifier (ID) to the controller of the new cell. The controller of the new cell may be the same as in the previous cell or it may be different from the previous controller handling the MS before the cell change. The MS ID comprises data required by the telecommunications system in order to be able to handle the incoming/outgoing call and signalling to/from the MS, respectively. Once the MS ID is received by the system, the system is aware of the current cell of the particular mobile station.
In time division multiple access (TDMA) networks, a plurality of time slots is used for the transmission between the mobile stations and the base transceiver stations. A frame is made up of a predetermined number of time slots. Each mobile station will be allocated a given time slot in each frame in which to communicate with a base transceiver station. In order to process correctly the signal received from various mobile stations, the signal from each mobile station must be received within its allocated time slot at the base station. To ensure that the signals are received within the allocated time slots the base station will provide the mobile station with timing advance (TA) information. The timing advance information indicates when the mobile station should transmit its signal to the base station. The timing advance information is required since propagation of radio waves between the transmitting and receiving stations causes a delay in the transmission between the stations. In other words, the travel of the radio waves from the BTS to the MS and vice versa takes some time, and thus there is a certain offset between the timings of the slots used by the MS and the BTS. In order to address the delay, and more precisely, to avoid any overlapping time slots, it is therefore necessary to determine the time delay between the mobile station and the serving base transceiver station.
According to an approach a mobile station in an idle mode adjusts its internal timing according to bursts received from the serving base station. When the MS transmits for the first time to the BTS (access bursts), this is done by using the internal timing of the mobile station which is delayed compared to BTS's timing. When the BTS receives these signals, it measures the time offset between its frame structure and the received MS bursts. The difference between these two timings equals to the propagation delay of the radio waves travelling from the BTS to the MS and back, i.e. twice the distance between the stations. To avoid overlapping time slots in the BTS, the BTS provides the MS with timing advance (TA) value information indicating the time how much earlier the MS should transmit towards the BTS. By means of this it is possible for the MS to adjust its transmission so that the transmission can be received by the BTS in slots according to the frame structure used by the receiving BTS. During a dedicated connection the BTS continuously determines the appropriate TA value and signals the TA value to the MS.
One possible way to determine the timing is based on impulse responses. The impulses response can be obtained by correlating a known bit pattern in a burst (a so called training sequence) with the received signal. The impulse response is a curve with the time on X-axis (component taps) and intensity on the Y-axis (see FIG. 3 showing an exemplifying impulse response). For normal operation of a TDMA system, such as the GSM, it is important that the reception is adjusted so that a maximum energy will be received. Thus the timing has been determined in the prior art from the mass center of the impulse response.
The functionalities of the cellular communication system facilitate a provision of an at least rough location estimate concerning the current location of an individual mobile station. More particularly, the cellular communication system is always aware (at least roughly) of the current location area of such mobile stations which are communicating with at least one of the base stations of the system. This information is available even when the mobile station is located within a coverage area of a visited or “foreign” network, as the visited network is capable of transmitting the location information of the mobile station back to the home network, e.g. for the purposes of routing and charging.
A location service feature provided by means of a cellular telecommunications network has been proposed. The proposed location service can provide the last known location of a mobile station together with a time-stamp. The proposed service functionality can be provided by a separate network element or server which receives the information from the various controllers of the system.
One possibility of making the location determination more accurate is to utilize the timing delay or offset information between a mobile station and a base station. This is based on the realization that when the mobile station is relatively far from the base station, the signal will travel for a longer time to the base station as compared to when the mobile station is relatively close to the base station. Therefore, in order to address the offset, a mobile station relatively far from the base station will have to send its signal to the base station more “in advance” relative to the timing structure than a mobile station which is relatively close to the base station. The timing advance information (TA) is thus also a measure of the distance between the mobile station and the base station. The timing advance information does not provide indication as to the direction from which the signal has been received. Thus the use of the timing advance information may require use of some other supporting information so that the direction can be estimated. For instance, the location may be accomplished such that the mobile station communicates also with at least one other and preferably with three neighboring base stations covering the area in which the mobile station is currently located in the event that no other indication of the direction is available. Directional antennae or Angle of Arrival (AOA) measurement from one base station site may also be used together with the timing advance information. In the latter case the location is determined to the intersection of AOA line and timing advance circle.
Currently the timing advance is determined e.g. in the GSM system with an accuracy of 1 bit. This corresponds to 3.69 μs in time and 1.1 km in distance. This accuracy is enough for the normal operation of the cellular communication system. However, it may not be enough for some special purposes, like for estimating a distance between two or several stations of the communication system and for geographically locating a mobile station based on the timing advance information.